1. Field of the Invention
The present invention relates to a focus inspection device and a focusing method using the focus inspection device, especially to those applied to a printing lens that is used for projecting an optical image of an original, like a picture frame on a photographic film or a photo-print onto a photosensitive recording material, like photographic paper.
2. Background Arts
As a method of adjusting the focus of the printing lens in photographic printers, it has been widely conducted making a test print from a test original that is photographed on a photographic film and contains an ordinary image, like a grayscale image of a landscape, and inspecting the test print with naked eyes. In that case, it is usual to make a plurality of test prints from the same original while shifting the focus of the printing lens step by step, and compare the test prints to each other for determining the optimum focal position. Which is called a round exposure method, and is disclosed for example in JPA Nos. 64-17013 and 01-200344.
According to this conventional method, however, the inspector cannot exactly determine the optimum focal position because it is not easy to judge the sharpness of the test print containing the ordinary image. Particularly when the test prints are made at a low print magnification, that is, when the printing lens is set at a low magnification and thus has a large depth of field, it is still more difficult to judge from the test print as to whether a reference focal plane, in which the test original and other originals to print are positioned, coincides with the center of the depth of field, i.e. the focal plane of the printing lens in the object side. If the reference focal plane does not coincides with the center of the depth of field of the printing lens, and the original is not held flat in the reference focal plane, the printed image can be blurred.
In view of the foregoing, an object of the present invention is to provide a focus inspection device for use in focusing an image forming lens, that facilitates to inspecting focal conditions of the image forming lens.
Another object of the present invention is to provide a focus inspection device that permits quantitative determination of a deviation of a focal plane of an image forming lens from a reference focal plane.
A further object of the present invention is to provide a focusing method for a printing lens, using such a focus inspection device.
According to the present invention, a focus inspection device for a lens that is determined to form an optical image of an original on a predetermined image forming surface when the original is placed in a predetermined reference focal plane that is perpendicular to an optical axis of the lens, comprises a focusing chart having a test pattern thereon; and a holding device for holding the focusing chart in a position inclined to the reference focal plane, wherein a deviation of an object focal plane of the lens from the reference focal plane is detected in view of sharpness of the test pattern on an image formed from the focusing chart through the lens.
The holding device preferably holds the focusing chart such that a center of the test pattern is aligned with the reference focal plane.
According to a preferred embodiment, the focusing chart further comprises a scale provided adjacent the test pattern along the inclined direction of the focusing chart, the scale serving as a measure of relative height of the inclined focusing chart to the reference focal plane.
The sharpness of the test pattern may be evaluated as density distribution on the test print, to detect the deviation value of the object focal plane of the printing lens as a deviation value of the lowest density area from the center of the test pattern on the test print.
The test pattern preferably comprises a plurality of lines extending in parallel to a transverse direction to the inclined direction of the focusing chart, the lines being equally spaced from each other in the inclined direction.
According to a more preferred embodiment, the test pattern comprises a plurality of rows of lines extending in parallel to a transverse direction to the inclined direction of the focusing chart, the lines being equally spaced from each other in the inclined direction within each row, and the spacing between the lines increases sequentially from one row to another.
Since the narrower spacing between the lines on the focusing chart results the narrower lowest density area on the test print, the lowest density area appearing on the reproduced test pattern consisting of the plurality of rows of lines with gradually increased line densities displays a substantially triangular shape. Therefore, it becomes possible to determine the deviation of the lowest density area from the center of the test pattern as a definite quantitative value.
According to another aspect of the present invention, a focusing method for a printing lens that is determined to print an image of an original on a photosensitive recording material placed in a predetermined position when the original is placed in a predetermined reference focal plane that is perpendicular to an optical axis of the lens, the method comprising the steps of: holding a focusing chart with a test pattern thereon in a position where the focusing chart is inclined to the reference focal plane with a center of the test pattern aligned with the reference focal plane; making at least a test print from the focusing chart through the printing lens; detecting a deviation value of an object focal plane of the printing lens from the reference focal plane in view of sharpness of the test pattern reproduced on the test print; and adjusting the object focal plane of the printing lens to the reference focal plane in accordance with the detected deviation value of the object focal plane.
It is preferable to make test prints from the focusing chart through the printing lens while setting the printing lens at different printing magnifications, detect a deviation value of an object focal plane of the printing lens from the reference focal plane at each of the different printing magnifications in view of sharpness of the test pattern reproduced on the test prints, and calculate correction amounts from the deviation values detected from the test prints, for focusing the printing lens in accordance with the correction amount.